Method of illuminating a photomask using chevron illumination

ABSTRACT

A microlithograpic tool, such as a projection stepper, for manufacturing integrated circuits, shapes light that illuminates a photomask with a chevron illumination system. The system uses either a chevron aperture mask of diffractive optical elements to shape a light source into four chevrons ( 110   b,    120   b,    130   b,    140   b ). The chevrons are located in the corners of the circular pupil of the condenser lens. The chevrons may be a small a square poles at the corners or as large as an annular square ring. The chevrons provide superior performance for illuminating conventional X and Y oriented features of a photomask.

CROSS-REFERENCE TO RELTED APPLICATIONS

[0001] This application claims priority of U.S. Provisional PatentApplication Serial No. 60/216,881 filed Jul. 7, 2000 (Attorney DocketNo. 88405.114400).

FIELD OF INVENTION

[0002] The present invention relates to the field of semiconductorlithography, more specifically to the illumination of a photomask forimaging improvement.

BACKGROUND OF THE INVENTION

[0003] Semiconductor lithography utilizing optical imaging systems hasbeen carried out for many years. The process involves the creation ofrelief image patterns through the projection of radiation within or nearthe UV visible portion of the electromagnetic spectrum. Earlier methodsof optical semiconductor lithography utilized a proximity printingtechnique, where a photomask with the desired device pattern image washeld close to the surface of a photosensitized silicon wafer surface,transferring the image the image to the surface. Resolution, devicesize, and device yield are limited using this approach because of thelack of reduction optics. Modern reduction projection techniques usingstep-and-repeat or step-and-scan optical systems minimize some of theproblems encountered with earlier proximity lithography methods and havelead to the development of tools that currently allow resolution below0.15 μm.

[0004] Semiconductor device features are generally on the order of thewavelength of the ultraviolet (UV) radiation used to pattern them.Currently, exposure wavelengths are on the order of 150 to 450 nm andmore specifically 157 nm, 293 nm, 248 nm, 365 nm, and 436 nm. The mostchallenging lithographic features are those which fall near or belowsizes corresponding to 0.5λ/NA, where λ is the exposing wavelength andNA is the objective lens numerical aperture of the exposure tool. As anexample, for a 193 nm-wavelength exposure system incorporating a 0.65NAobjective lens, the imaging of features at or below 0.13 micrometers isconsidered state of the art. Generally, systems employ Köhler typeillumination and an effective source that is shaped circularly. Morerecently, source shapes have been varied from this conventional circularshape to best optimize illumination conditions for a specific photomaskpattern, wavelength, NA, and other imaging parameters. Off axisillumination using dipole illumination, with a pair or circular sourceshapes oriented in the direction of mask geometry can offer asignificant enhancement to imaging performance. This is because onlyoblique illumination at an optimized illumination angle can be designedto allow projection of a single orientation of mask diffraction energyat the outermost edges of an objective lens pupil. The problem withdipole illumination arises when geometry of both X and Y (or horizontaland vertical) nature is considered since imaging is limited to featuresoriented along one direction in an X-Y plane. Additionally, the use ofcircular pole shapes could be improved by using poles with square orrectangular shaping. Four pole or quadrupole source shapes are anexample of a modification for X and Y oriented geometry [see forinstance U.S. Pat. No. 5,305,054]. Here, four circular poles areutilized to accommodate the mask geometry located along two orthogonalaxes. The use of multiple circular shaped poles is not the best shapingfor use with mask geometry oriented on orthogonal X and Y-axes however.I have discovered that particular square pole shapes extended along Xand Y axes, forming chevron shapes at the corners of an illuminationsource, show superior performance to other illumination approaches.

SUMMARY OF THE INVENTION

[0005] The present invention is a unique approach to shapingillumination. It can be implemented by placing an aperture in the pupilplane of the condenser lens or illumination system or by shapingillumination using optical means including diffractive optical elements(DOEs) or other similar elements, beam splitters or other similarmethods.

[0006] One object of the present invention is to provide off-axisillumination that is optimized for mask features oriented alongorthogonal X and Y-axes as most semiconductor devise geometry isoriented along these directions. A second object of this invention is toremove the illumination source shaping that is not optimal forsemiconductor device geometry oriented along orthogonal X and Y-axes.

[0007] Another object of this invention is to allow for the control ofthe ratio of on-axis to off-axis illumination shaping by means ofmodification to the shape of the illumination source along X and Y axes.

[0008] Another object of the present invention is to provideillumination shaping that can be implemented in a projection imagingsystem through various means, including the use of diffractive opticalelements (DOEs), beam splitters, aperture plates or masks, or otheroptical means.

[0009] These objects are achieved using a particular illuminationshaping where four regions of illumination are provided and theseregions are defined by boundaries that lie on X and Y axes. Morespecifically, these regions are “L” shaped, where the horizontal andvertical segments are varied in size between two limits. The first limitis where the horizontal and vertical segments of the four “L” shapesform squares, where the length and width of the two segments are equal.The second limit is where the length of the horizontal and verticalsegments of the four “L” shapes is such that each form a connection withthe neighboring “L” shapes in horizontal and vertical directions,resulting in a continuous square ring of illumination.

[0010] The aperture mask of the invention may be formed on a translucentor transparent substrate, such as quartz, with chevron openings in anopaque coating or from a solid, opaque metal plate with chevron openingsin the plate. The chevrons are located in the four corner regions of theplate. Each chevron includes a square region located in the corner andfirst and second legs extending from said square transparent region andin a direction toward adjacent corners. The chevrons may be varied insize from as small as four square corners to as large as a squareannular ring around a central opaque region.

BRIEF DESCRIPTION OF THE FIGURES

[0011]FIGS. 1a-1 e show Chevron illumination distributions withincreasing edge obscuration, FIG. 1a, none or square ring, FIG. 1b,0.25, FIG. 1c, 0.50, FIG. 1d, 0.75, and FIG. 1e, 1.0 or squarequadrupole.

[0012]FIG. 2 shows the implementation of the invention in a projectionlithography tool using an aperture mask in the illuminator.

[0013]FIG. 3 is a beam shaping method for chevron-shaped illumination.

[0014]FIG. 4 shows results of image evaluation using NILS vs. focus for130 nm lines on various pitch values using the ASML-Quasar(TM)illumination, with parameters (outer sigma/inner sigma/opening angle) of0.8/0.5/30°.

[0015]FIG. 5 shows results of image evaluation using NILS vs. focus for130 nm lines on various pitch values using the chevron “e” illuminator.

[0016]FIG. 6 shows results of image evaluation using NILS vs. focus for130 nm lines on various pitch values using the chevron “d” illuminator.

[0017]FIG. 7 shows results of image evaluation using NILS vs. focus for130 nm lines on various pitch values using the chevron “c” illuminator.

[0018]FIG. 8 shows results of image evaluation using NILS vs. focus for130 nm lines on various pitch values using the chevron “b” illuminator.

[0019]FIG. 9 shows results of image evaluation using NILS vs. focus for130 nm lines on various pitch values using the Chevron “a” illuminator.

DESCRIPTION OF THE INVENTION

[0020] The present illumination method is referred to as chevronillumination. Examples of chevron illumination plates are shown theFIGS. 1a-1 e. The chevrons are formed and sized to fit into a circularpupil of a conventional illumination system. FIGS. 1a-1 e show severalexamples from a full square ring to four square poles. The solutionsbetween these extremes are the Chevron shapes. These shapes showimprovement beyond either the quadrupole or the ASML-Quasar(TM) shapingcurrently utilized in projection systems.

[0021] In one embodiment of the invention the illumination plates areformed by conventional photomask technology. A transparent quartzsubstrate is covered with an opaque coating, typically chrome. Thesubstrate is covered with a photoresist and the chevron patterns areexposed in the resist. Exposure is typically by an electron beamapparatus is a manner well know to those skilled in the art. In anotherembodiment, a solid metal plate is fabricated to have chevron openingsin its corners.

[0022] Turning to FIG. 1b, there is shown an aperture mask with fourchevrons 110 b, 120 b, 130 b and 140 b. The chevrons have respectivepairs of legs 111 b, 112 b, 121 b, 122 b, 131 b, 131 b, and 141 b, 142b. The legs extend toward adjacent corners. The chevrons in successiveFIGS. 1c-1 e become progressively smaller until the chevrons 120 e, 130e, 140 e and 150 e in FIG. 1e are simple transparent squares in thechrome covering of a quartz substrate or are openings in the metalsubstrate. In FIG. 1a, the chevrons meet between the adjacent corners toform an annular rectangular or square ring 100.

[0023] Those skilled in the art understand that the preferredembodiments have openings with equal dimensions in both horizontal (X)and vertical (Y) directions. In that way, the source image is equallyshaped in both its X and Y dimensions. However, if desired, the openingscould be different in the X and Y directions to emphasize one dimensionover the other. Köhler illumination systems are used extensively inlithographic applications and are well known. The present invention isimplemented in existing Köhler illumination systems via access to theillumination optical system. One example is shown in FIG. 2. Here, alight source 90 generates a light beam that is directed through acondenser lens system 97. Within this system, an aperture mask 91 isplaced in the condenser lens pupil plane. The mask 91 controls the shapeof the light beam into the chevron-shaped intensity distribution. Thecondenser lens focuses an image of the chevron shaped light source onthe pupil plane 96 of the objective lens system 93. The photomask 92 ispositioned at the object plane of the objective lens system 93 and isilluminated by the chevron shaped light source image. An image of thephotomask 92 is projected through the objective lens system 93 towardits image plane 95. The aperture mask may be either a chrome coveredsubstrate with chevron openings in the chrome or a solid, opaquesubstrate with chevron openings.

[0024]FIG. 3 shows another example with a beam-shaping optical system ofan illumination system that is modified to produce the desiredchevron-shaped illumination. The illumination optical system contains abeam shaping optical system 52 and optical integrator 60. A light sourcesuch as a lamp or an excimer laser 51 is used for illumination. The beamshaping optical system 52 shapes the light beam coming from source 51into the chevron shape and directs this shaped beam onto the surface ofan optical integrator 60. The optical integrator can consist of, forexample, a fly's eye array or one or more diffractive optical elements.The condenser lens system 53 illuminates the mask with Köhlerillumination. The intensity distribution in the mask plane 54 is aresult of the chevron shaping of the illuminator and is the FourierTransform of the illuminated shape. An image of the chevron sourceshaping is also produced in the objective lens system 56-pupil plane 55.Diffractive optical elements (DOEs), also known as binary opticalelements (BOEs), are often employed in the illumination systems oflithographic tools as beam shaping components. A diffractive opticalelement operates on the principle of diffraction. Traditional opticalelements use their shape to bend light. Diffractive optics work asFourier Transform lenses to form desired optical effects. DOE patterntechnology produces multiple phase levels by using ion etching methods,resulting in deflection angles large enough to allow for shaping that isgenerally circular in nature. The use of such elements in lithographicsystems can be found for instance in U.S. Pat. No. 5,926,257 where a setof DOEs is used to form circular Köhler illumination. In U.S. Pat. No.5,631,72, an array of diffractive optical elements is placed on or nearthe focal point of the condenser to generate a desired circular angulardistribution with little dependence on the illumination source profile.Fabrication methods such as those described in U.S. Pat. No. 6,120,950and U.S. Pat. No. 5,227,915 are well known to those of ordinary skill inthe art of diffractive optics. The beam shaping optical system 52 cancontain one or more diffractive optical elements to achieve the chevronshaping using these fabrication methods.

[0025] Imaging results using the present invention are presented. Thefive chevron-shaped illumination shapes used are those shown in FIG. 1.The five designs (a through e) have increasing edge obscuration, fromnone or zero to full or 1.0, where zero obscuration is a full ring and1.0 is a square quadrupole. The width of the segments of the fourillumination regions is such that the outside edge of the illuminationregions extend to 0.7 of a full unity sigma pupil and the inside edge is0.5 of a full pupil. The intensity in the open areas of the patterns is1.0 and the intensity in the neighboring region is zero. Imageevaluation has been carried out for an imaging system with an objectivelens NA of 0.75 and a wavelength of 248 nm for 130 nm line features withpitch values from 1:1 to 1:4, corresponding to 260 nm to 650 nm pitchvalues respectively. Evaluation has been carried out using a vectoraerial image model to incorporate imaging parameters. The metric ofevaluation using the slope of the logarithm of the intensity image(known as the aerial image) has been used. A sharper image is one thatpossesses a larger value of this metric. Evaluation has been performedfor imaging though a range of focus values. It is desirable that sharpimages are produced though as large a variation of focus as possible.Imaging results using the five variations on the chevron shapedillumination have been compared to those for a quadrupole illumination,known as ASML Quasar(TM) illumination, defined with an inner sigma valueof 0.5, an outer sigma value of 0.8 and an arc definition of 30°.Results are shown in FIGS. 4 through 9. The imaging performance aresummarized as follows.

[0026] The through-pitch NILS vs. focus (where NILS is defined as theproduct of feature size and the slope of the log of the aerial image,and where larger NILS values are desired) for the ASML-Quasar(TM) designevaluated in FIG. 4 show how NILS values above 1.5 can be achievedthrough a defocus near 0.25 microns, but the difference between isolatedand dense feature performance (referred to as proximity bias) issignificant and may not be desirable

[0027] The performance of the chevron shaping shown in FIG. 1e is shownin FIG. 5, where the smallest pitch features is improved over theASML-Quasar(TM) but proximity bias is increased. This shows how thisvariation to the chevron-shaped illumination can be used for imageimprovement through a large variation in focus if proximity bias is nota concern.

[0028]FIG. 6 shows the results for chevron design of FIG. 1d where the260 nm pitch performance is better than that for the ASML-Quasar(TM) andproximity bias is reduced over FIG. 5. This shows how the control overthe specific chevron shaping can lead to controlled variation in imagingperformance through focus and proximity bias.

[0029]FIG. 7 shows the best overall performance of the chevron is forthe chevron of FIG. 1c. The NILS through focus is superior to theASML-Quasar(TM) and the proximity bias is reduced. This is the superiorresult.

[0030]FIG. 8 shows the result for the chevron design of FIG. 1b. Itdemonstrates how the chevron shaping can allow for control overproximity bias and performance, which can allow the source shaping to betailored to specific imaging and process requirements.

[0031]FIG. 9 shows the result for the chevron design of FIG. 1a, whichis a square ring. The square ring shows best isolated to dense processoverlap, or minimal proximity bias.

[0032] The results show the significance of the invention. When geometryis oriented along X/Y directions (as is the requirement for a quadrupoletype illumination application), it is most beneficial to use anillumination source which projects energy to X and Y axes. This isaccomplished with the chevron-shaped designs and cannot be accomplishedusing a circular quadrupole or ASML-Quasar(TM) illumination.

[0033] Although the present invention has been described, it is to beunderstood that it is not limited to these descriptive examples. Thedescribed embodiments are not necessarily exclusive and various changesand modifications in methods, designs, and placement may be made theretowithout departing from the scope of the invention as described here.

What I claim is:
 1. A photolithographic system for forming finely spacefeatures on a photosensitized surface of a semiconductor wafercomprising: a light source for illuminating a photomask covered with apattern that will be transferred to the photosensitized surface of thesemiconductor wafer; an aperture mask disposed between the light sourceand the semiconductor wafer and comprising a translucent substrate withan opaque coating and four transparent corner regions; each transparentcorner region comprising a chevron including a square transparent regionlocated in the corner and first and second legs extending from saidsquare transparent region and in a direction toward adjacent corners. 2.The photolithographic system of claim 1 wherein the legs of chevronsextend half the length of the distance between adjacent corners.
 3. Thephotolithographic system of claim 2 wherein transparent chevrons form arectangular annular transparent region surrounding an opaque rectangularcentral region.
 4. A photolithographic system for forming finely spacefeatures on a photosensitized surface of a semiconductor wafercomprising: an objective lens system having a pupil plane and an imageplane for focusing an image of a photomask pattern located at said pupilplane onto the sensitized surface of the semiconductor wafer located atsaid image plane; an illumination system having a light source and acondenser lens for focsing an image of the light source onto said pupilplane of said objective lens system; a diffractive optical elementsystem located between the light source and the condenser lens forshaping light from the light source into chevron image patterns, eachchevron image pattern located in a corner of a pupil plane of theillumination system and having first and second legs extending from saidcorner in a direction toward adjacent corners; and an optical integratordisposed between the diffractive optical element system and thesemiconductor wafer.
 5. The photolithographic system of claim 4 whereinthe legs of chevrons extend half the length of the distance betweenadjacent corners.
 6. The photolithographic system of claim 5 whereintransparent chevrons form a rectangular annular transparent regionsurrounding an opaque rectangular central region.
 7. A photolithographicsystem for forming finely space features on a photosensitized surface ofa semiconductor wafer comprising: an objective lens system having apupil plane and an image plane for focusing an image of a photomaskpattern located at said pupil plane onto the sensitized surface of thesemiconductor wafer located at said image plane; an illumination systemhaving a light source and a condenser lens for focsing an image of thelight source onto said pupil plane of said objective lens system; meansfor shaping light from the light source into chevron image patterns,each chevron image pattern located in a corner of a pupil plane of theillumination system and having first and second legs extending from saidcorner in a direction toward adjacent corners.
 8. The photolithographicsystem of claim 7 wherein the means for shaping light comprises adiffractive optical element system located between the light source andthe condenser lens for shaping light from the light source into chevronimage patterns, each chevron image pattern located in a corner of apupil plane of the illumination system and having first and second legsextending from said corner in a direction toward adjacent corners. 9.The photolithographic system of claim 8 wherein the means for shapinglight comprises an aperture mask disposed between the light source andthe condenser lens and comprising an opaque coating and four transparentcorner regions, each transparent corner region comprising a chevronincluding a square transparent region located in the corner and firstand second legs extending from said square transparent region and in adirection toward adjacent corners.
 10. A method for forming finely spacefeatures on a photosensitized surface of a semiconductor wafercomprising: providing a source of light; shaping light from the lightsource into chevron image patterns, each chevron image pattern locatedin a corner of a pupil plane of a condenser lens and having first andsecond legs extending from said corners in directions toward adjacentcorners; focusing the chevron images of the light source onto aphotomask disposed in a pupil plane of an objective lens system;focusing an image of the photomask pattern onto the sensitized surfaceof said semiconductor wafer located at an image plane of said objectivelens systems for transferring the pattern on said photomask to thesensitized surface of the semiconductor wafer.
 11. The method of claim10 wherein the step of shaping light omprises diffracting a light sourceinto chevron shaped images.
 12. The method of claim 11 wherein the stepof shaping light comprises inserting an aperture mask between the lightsource and the condenser lens and providing transparent chevron openingsin an opaque coating on the aperture mask.
 13. An aperture mask for aphotolithographic system comprising: a transparent substrate; an opaquecovering on said substrate; four transparent regions, each transparentregion located in one of four corners of said aperture mask.
 14. Theaperture mask of claim 13 wherein each of the four transparent regionscomprises a rectangular transparent region.
 15. The aperture mask ofclaim 14 wherein each of the rectangular transparent regions is a squaretransparent region.
 16. The aperture mask of claim 15 wherein eachtransparent region comprises a chevron including the square transparentregion and has first and second legs extending from said squaretransparent region and in a direction toward adjacent corners.
 17. Theaperture mask of claim 16 wherein the legs of all chevrons have the samelength.
 18. The aperture mask of claim 17 wherein the legs of chevronsextend half the length of the distance between adjacent corners.
 19. Theaperture mask of claim 18 wherein the transparent regions form arectangular annular transparent region surrounding an opaque rectangularcentral region.
 20. An aperture mask for a photolithographic systemcomprising: a mask with an opaque coating and four transparent cornerregions; each transparent corner region comprising a chevron including asquare transparent region located in the corner and first and secondlegs extending from said square transparent region and in a directiontoward adjacent corners.
 21. The aperture mask of claim 20 wherein thelegs of chevrons extend half the length of the distance between adjacentcorners.
 22. The aperture mask of claim 21 wherein the transparentregions form a rectangular annular transparent region surrounding anopaque rectangular central region.